Peak detecting bar code reader

Abstract

A signal processor for reading a bar code symbol having alternating dark and light portions includes a photodetector for receiving light reflected from the bar code symbol and for forming a sense signal representing the intensity of the reflected light. A differentiator circuit coupled to the photodiode generates a first derivative of the sensed signal, and an offset circuit generates an offset signal which is displaced slightly from the first derivative. An output circuit generates an output signal which indicates changes in the intensity of the reflected light when the first derivative signal and the offset signal have approximately equal amplitudes.

I claim: 
     
       1. A signal processor for reading a bar code symbol having alternating dark and light portions, the circuit comprising: a) photodetector means for receiving light reflected from the bar code symbol and for forming from the reflected light a sense signal representing the intensity of the reflected light;   b) differentiator circuit means, coupled to the photodetector means, for generating a first derivative signal representing a first derivative of the sensed signal;   c) signal shift means, coupled to the differentiator circuit means, for generating a shifted signal which is displaced from the first derivative signal, said shift signal having at least one substantially identical peak value as the first derivative signal; and   d) output means, coupled to the differentiator circuit means and the signal shift means, for generating an output signal indicating a change in the intensity of the reflected light when the first derivative signal and the shifted signal have approximately equal amplitudes.   
     
     
       2. The signal processor of claim 1 wherein the output means includes: a comparator for comparing the amplitudes of the first derivative signal and the shifted signal.   
     
     
       3. The signal processor of claim 1 wherein the signal shift means includes: means for delaying the-first derivative signal to produce the shifted signal.   
     
     
       4. The signal processor of claim 1 wherein the differentiator circuit means, the signal shift means, and the output means each includes an operational amplifier. 
     
     
       5. A signal processor for reading a bar code symbol having alternating dark and light portions, the circuit comprising: a) photodetector means for receiving light reflected from the bar code symbol and for forming from the reflected light a sensed signal representing the intensity of the reflected light;   b) differentiator circuit means, coupled to the photodetector means, for generating a first derivative signal representing a first derivative of the sensed signal;   c) noise removing means, coupled to the differentiator circuit means, for removing false transitions due to noise including noise having substantially the same frequency as the sensed signal; and   d) output means, coupled to the differentiator circuit means, for generating an output signal indicating a change in the intensity of the reflected light from the first derivative signal without forming a higher derivative of the sensed signal.   
     
     
       6. The signal processor of claim 5 wherein the output means includes: signal shift means, coupled to the differentiator circuit means, for generating a shifted signal which is displaced from the first derivative signal; and   a comparator for comparing the amplitudes of the first derivative signal and the shifted signal.   
     
     
       7. The signal processor of claim 6 wherein the signal shift means includes: means for delaying the first derivative signal to produce the shifted signal.   
     
     
       8. A method for reading a bar code symbol having alternating dark and light portions, the method comprising the steps of: a) receiving light reflected from the bar code symbol and forming from the reflected light a sense signal representing the intensity of the reflected light;   b) generating a first derivative signal representing a first derivative of the sensed signal;   c) generating a shifted signal which is displaced from the first derivative signal and having at least one substantially identical peak value as the first derivative signal; and   d) generating an output signal indicating a change in the intensity of the reflected light when the first derivative signal and the shifted signal have approximately equal amplitudes.   
     
     
       9. The method of claim 8 wherein the step of generating an output signal includes the step of: comparing the amplitudes of the first derivative signal and the shifted signal.   
     
     
       10. The method of claim 8 wherein the step of generating shifted signal includes the step of: delaying the first derivative signal to produce the shifted signal.   
     
     
       11. A signal processor for reading a bar code symbol having alternating dark and light portions, the circuit comprising: a) photodetector means for receiving light reflected from the bar code symbol and for forming from the reflected light a sensed signal representing the intensity of the reflected light;   b) differentiator circuit means, coupled to the photodetector means, for generating a first derivative signal representing a first derivative of the sensed signal;   c) signal shift means, coupled to the differentiator circuit means, for generating a shifted signal which is displaced from the first derivative signal; and   d) output means, coupled to the differentiator circuit means and the signal shift means, for generating an output signal indicating a change in the intensity of the reflected light when the first derivative signal and the shifted signal have approximately equal amplitudes, wherein the output means includes signal error detector means for identifying as signal errors changes in the intensity of the reflected light which do not exceed a predetermined threshold, and signal error correction means, coupled to the signal error detector means, for preventing the output means from indicating the presence of a change in the intensity of the reflected light when the signal error detector means identifies signal errors.       
     
     
       12. A method for reading a bar code symbol having alternating dark and light portions, the method comprising the steps of: a) receiving light reflected from the bar code symbol and forming from the reflected light a sense signal representing the intensity of the reflected light;   b) generating a first derivative signal representing a first derivative of the sensed signal;   c) generating a shifted signal which is displaced from the first derivative signal; and   d) generating an output signal indicating a change in the intensity of the reflected light when the first derivative signal and the shifted signal have approximately equal amplitudes, wherein the output signal generating step includes the steps of: identifying as false readings changes in the intensity of the reflected light which do not exceed a predetermined threshold, and   preventing the output means from indicating the presence of a change in the intensity of the reflected light when a false reading is identified.     
     
     
       13. A bar code symbol reader for reading a symbol having alternating dark and light portions defining edges, the reader comprising: means for reflecting light from the bar code symbol;   photodetector means for receiving the light reflected from the bar code symbol and for forming from the reflected light a sensed analog signal representing an intensity of the reflected light;   differentiator circuit means, coupled to the photodetector means, for generating a first derivative signal representing a first derivative of the sensed analog signal, the first derivative signal having peak amplitudes corresponding to the edges;   signal delay means, coupled to the differentiator circuit means, for generating a signal delayed from the first derivative signal by a predetermined time delay; and   output means, coupled to the differentiator circuit means and the signal delay means, for generating an output signal indicating a change in the intensity of the reflected light corresponding to a crosspoint of the first derivative signal and the delayed signal.